1. Field of the Invention
The present invention relates to a method for determining the phase of information modulated in a code-modulated signal transmitted by a satellite, using a receiver. At least some of the same information is transmitted from at least a first and a second satellite substantially simultaneously and the code-modulated signal transmitted from at least the first and the second satellite is received.
The invention also relates to a positioning system including at least two satellites, a positioning receiver, and means, in the receiver, for determining the phase of information modulated in a code-modulated signal transmitted by the satellites. In the positioning system at least partly the same information is arranged to be transmitted from first and second satellites substantially simultaneously. The receiver includes at least means for receiving the code-modulated signal transmitted from the first and the second satellite.
The invention further relates to a positioning receiver including means for receiving a code-modulated signal transmitted from at least a first and a second satellite, where at least partly the same information is being transmitted in a code-modulated signal from the first and the second satellite substantially simultaneously. The positioning receiver also includes means for determining the phase of information modulated in the code-modulated signal transmitted from the satellites.
The invention still further relates to an electronic device including a positioning receiver for receiving a code-modulated signal transmitted from at least a first and a second satellite, at least partly the same information being transmitted in a code-modulated signal from the first and the second satellite substantially simultaneously. The electronic device further includes means for determining the phase of information modulated in the code-modulated signal transmitted from the satellites.
2. Discussion of the Related Art
One known positioning system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites, of which a maximum of 12 are simultaneously within the sight of a receiver. These satellites transmit e.g. Ephemeris data of the satellite, as well as data on the time of the satellite. A receiver used in positioning normally deduces its position by calculating the propagation time of a signal transmitted simultaneously from several satellites belonging to the positioning system to the receiver. For the positioning, the receiver must typically receive the signal of at least four satellites within sight to compute the position.
Each satellite of the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L1. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, the modulation of these signals is performed with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to distinguish between the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with a polynome X10+X3+1, and the second binary sequence G2 is formed by delaying the polynome X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the code epoch is 1 ms. The L1 carrier signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the health of the satellite, its orbit, time data, etc.
During their operation, the satellites monitor the condition of their equipment. The satellites may use for example so-called watch-dog operations to detect and report possible faults in the equipment. The errors and malfunctions can be instantaneous or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, different satellites can be weighted differently on the basis of the health data. Thus, it is possible to minimize the effect of errors on measurements, possibly caused by satellites which seem unreliable.
To detect the signals of the satellites and to identify the satellites, the receiver must perform acquisition, whereby the receiver searches for the signal of each satellite at the time and attempts to be synchronized and locked to this signal so that the data transmitted with the signal can be received and demodulated. After the acquisition, the receiver attempts to keep locked, or to track the signal of the satellite at least during the time of positioning, but in some cases, the tracking phase can be maintained as long as the receiver receives the signal of the satellite sufficiently strongly.
The positioning receiver must perform the acquisition e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. Such a situation can easily occur e.g. in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which impairs the strength of the signal coming in the receiver. Also, in urban areas, buildings affect the signal to be received, and furthermore, so-called multipath propagation can occur, wherein the transmitted signal comes into the receiver along different paths, e.g. directly from the satellite (direct line-of-sight) and also reflected from buildings. This multipath propagation causes that the same signal is received as several signals with different phases.
The positioning arrangement has two primary functions:
1. to calculate the pseudo range between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudo ranges and the position data of the satellites. The position data of the satellites at each time can be calculated on the basis of the Ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudo ranges, because the time is not accurately known in the receiver. Thus, the determinations of position and time are iterated until a sufficient accuracy is achieved with respect to time and position. Because time is not known with absolute precision, the position and the time must be determined e.g. by linearizing a set of equations for each new iteration.
The pseudo range can be calculated by measuring the pseudo transmission time delays between signals of different satellites. After the receiver has been synchronized with the received signal, the information transmitted in the signal is determined.
FIG. 1b shows, in a reduced principle view, the determination of the code phase and the frequency deviation on the basis of the received signal by dividing a two-dimensional code phase and frequency space into cells C11, C12, . . . , C1n, . . . , Cm1, . . . , Cmn. The code phase is shown in the horizontal direction and the frequency deviation in the vertical direction. One cell E to be searched is shown darker, and an advantageous direction of searching is illustrated with an arrow D. In this example, one square, in the horizontal direction, indicates a half bit and, in the vertical direction, a frequency range xcex94f to be examined at a time. It is obvious that the chart shown in the example of FIG. 1b is considerably reduced. In practice, there may be cells to be searched on as many as 2046 time levels and on 20 freqency levels, wherein the space to be searched comprises 40,920 cells. If it takes e.g. approximately 1 ms to examine one cell, this means in practice that it takes more than 40 seconds to scan the whole space to be searched.
Almost all known GPS receivers utilize correlation methods for calculating the distances. In a positioning receiver, reference codes ref(k), i.e. the pseudo random sequences for different satellites are stored or generated locally. A received signal is subjected to conversion to an intermediate frequency (down conversion), after which the receiver multiplies the received signal with the stored or replicated pseudo random sequence. The signal obtained as a result of the multiplication is integrated or low-pass filtered, wherein the result is data about whether the received signal contained a signal transmitted by a satellite. The multiplication is iterated in the receiver so that each time, the phase of the pseudo random sequence stored in the receiver is shifted. The correct code phase is inferred from the correlation result preferably so that when the correlation result is the greatest, the correct phase has been found. Thus, the receiver is correctly synchronized with the received signal. However, this method is relatively slow, particularly at weak signal strengths, since the weaker the signal to be examined is, the more code epochs must be used in computing the correlation. In some receivers of prior art, the acquisition of the receiver has been made faster by increasing the number of correlators. For example, 36 correlators have been used in a 12-channel receiver, and in some cases even 240 correlators are in use. Thus, the searching may become faster, but on the other hand, the arrangement makes the structure of the positioning receiver more complicated and may significantly increase the power consumption.
Under poor signal conditions, the level of the received signal is so weak that it is not easy to find out the data transmitted in the signal from the signal of one code epoch (approx. 1 ms). An attempt can be made to increase the signal to be used for correlation to multiples of code epochs, i.e. to several milliseconds. However, the extension of the correlation time is limited by the accuracy of the local oscillator of the receiver as well as by the fact that navigation data is modulated in the signal. If the accuracy of the local oscillator is not sufficient, it is not possible to integrate the signal received during the time of several code epochs in a coherent way. Incoherent integration does, in turn, not improve the signal to noise ratio as much as could be possible to achieve by coherent integration. In practice, receivers can achieve receiving times not greater than in the order of 20 ms in coherent integration. The navigation data modulated in the signal will cause that if the points of change in the data are not known, coherent integration is not useful, since the bit can change during the integration. Thus, if several code epochs are utilized in the correlation, the point of bit change in the navigation data should be found out, after which only such code epochs can be used in the correlation in which the bit value is the same, i.e. the code used in the modulation has the same phase throughout the correlation time. This should be considered both at the stage of acquisition and at the stage of tracking.
After the code acquisition has been completed, the next steps are frequency tuning and phase locking. This correlation result also indicates the information transmitted in the GPS signal.
The above-mentioned acquisition and frequency control process must be performed for each signal of a satellite received in the receiver. Some receivers may have several receiving channels, wherein an attempt is made on each receiving channel to be synchronized with the signal of one satellite at a time and to find out the information transmitted by this satellite.
A positioning receiver receives information transmitted by satellites and performs positioning on the basis of the received information. For the positioning, the receiver must receive the signal transmitted by at least four different satellites to find out the x, y, z coordinates and the time data, if none of this information is available for use by the receiver in a sufficiently reliable way. In some cases, it is possible to transmit, e.g. from a base transceiver station, the height data of the base station, wherein for the positioning it is sufficient that the receiver receives the signal transmitted by three satellites. Inaccuracies of a few meters in the height direction do not significantly impair the positioning accuracy. The received navigation information is stored in a memory, wherein of this stored information e.g. Ephemeris data of satellites can be used.
FIG. 1a shows, in a principle chart, positioning by means of a signal transmitted by four satellites SV1, SV2, SV3, SV4, and a reference receiver BS, in a positioning receiver MS. In the GPS system, satellites transmit Ephemeris data and time data, which can be used in the positioning receiver for computing to determine the position of the satellite at the time. These Ephemeris data and time data are transmitted in frames which are further divided into subframes. FIG. 2 shows an example of such a frame structure FR. In the GPS system, each frame comprises 1500 bits which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe thus takes 6 s, and the whole frame is transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each subframe 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information about the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for the transmission of Ephemeris data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data on all the satellites. The entity of these subframes and frames is called a GPS navigation message which comprises 25 frames, or 125 subframes. The length of the navigation message is thus 12 min 30 s.
In the GPS system, time is measured in seconds from the beginning of a week. In the GPS system, the moment of beginning of a week is midnight between a Saturday and a Sunday. Each subframe to be transmitted contains information on the moment of the GPS week when the subframe was transmitted. Thus, the time data indicates the moment of transmission of a certain bit, i.e. in the GPS system, the moment of transmission of the last bit in the subframe. In the satellites, time is measured with high-precision atomic chronometers. In spite of this, the operation of each satellite is controlled in a control centre for the GPS system (not shown), and e.g. a time comparison is performed to detect chronometric errors in the satellites and to transmit this information to the satellite.
In the receiver, the moment of transmission {circumflex over (T)}ToAk of the received signal can be determined for example in the following way:                                           T            ^                    ToA          k                =                              TOW            k                    +                      T                          m              ⁢                              xe2x80x83                            ⁢              s                        k                    +                      T            chip            k                    +                      T                          Δ              ⁢                              xe2x80x83                            ⁢              chip                        k                                              (        1        )            
in which
TOWk=the time data (time of week) contained in the last received subframe,
Tmsk=the time passed since the reception of the last received bit corresponding to the number of bits received after the last bit of the bit corresponding to the time data, i.e. in the GPS system the last bit of the last received subframe containing the time data,
Tchipk=the number (from 0 to 1022) of whole chips received after the change of the last epoch,
Txcex94chipk=the code phase measured at the time of positioning, and
k=the satellite index.
All the terms of Formula (1) to be added can be given in units of time (seconds). Further, the length of the chips and bits in time is known and it is substantially constant. As can be seen from Formula (1), only the last two terms in the determination of the moment of receiving a signal are related to the received signal as such. The other terms are related to information transmitted in this signal, and they are measured in relation to the received navigation data and the local reference time of the receiver.
The appended FIG. 3 illustrates this formula and its different terms, used for estimating the moment of receiving a signal received at a moment of positioning. It is obvious that FIG. 3 is simplified with respect to the real situation, because e.g. one code epoch comprises 1023 chips, wherein it is not reasonable to illustrate them in detail. The moment of positioning is illustrated by a dash-and-dot line indicated with the reference SM.
The measurement of the last two terms in Formula (1) requires that the receiver is properly synchronized and locked to this signal. It is thus possible in the receiver to determine each chip and its phase by using a satellite reference code stored or generated in the receiver, and a code phase loop.
It is important to calculate the moment of receiving the received signal for each signal to be tracked, because the local reference time of the receiver, formed by the local oscillator of the receiver, is coupled to the GPS time on the basis of these values. Furthermore, the different propagation times of signals received from different satellites can be deduced from these measured values, because each satellite transmits the same chip substantially at the same moment. Even though there may be minor differences in the timings of different satellites, they are monitored, and the error data is transmitted in the GPS navigation message, as was already mentioned above.
Under good receiving conditions and upon using an advantageous satellite constellation, the user""s position and time error can be solved very accurately. A good satellite constellation means that the satellites to be used for positioning are selected so that seen from the receiver, they are clearly located in different directions, or the space angles at which the signals transmitted from different satellites arrive at the receiver are clearly different.
It is an aim of the present invention to provide a method for determining the position of a receiver also when the signal strength is so weak that navigation information cannot be received from the necessary four satellites. It is also an aim of the invention to provide a positioning receiver. The invention is based on the idea that in the positioning, the moment of bit change is determined preferably by means of one satellite with a signal which is received at a sufficient strength. It is thus possible to determine the moment of bit change from the signals of the other satellites on the basis of the relative differences in the times of receiving the signals from the different satellites, as well as almanac data, the estimated position of the receiver, and the reference time. The estimate used for the position of the receiver can be the position of the base transceiver station in whose operating range the receiver is located, i.e. the position of the so-called serving base station.
The method according to the present invention includes a first acquisition step, in which the receiver is synchronized with the signal of at least the first satellite, a determination step, in which a moment of a bit change of an information data bit in the modulated signal of the first satellite is used to determine the phase of information modulated in the signal, a computing step to compute the difference in propagation time of the signal transmitted by said first satellite and the signal transmitted by the second satellite from the satellite to the receiver, a second acquisition step, in which the receiver is synchronized with the signal of the second satellite on the basis of the difference in propagation time computed in the computing step, and an integration step to receive the signal of the second satellite and to integrate sequences of a certain length to improve the demodulability of the signal.
The positioning system according to the present inventionxe2x80x94includes a first acquisition means for synchronizing the receiver at least with the signal of the first satellite, determining means for determining the phase of the information modulated in the signal of the first satellite on the basis of a moment of a bit change of an information data bit of the modulated signal, computing means for computing the difference in the time of propagation from the satellite to the receiver between the signal transmitted from the first satellite and the signal transmitted from the second satellite, second acquisition means for synchronizing the receiver with the signal of the second satellite on the basis of the difference in propagation time computed in the computing means, and integrating means for integrating sequences of a certain length from the received signal of the second satellite to improve demodulability of the signal.
The positioning receiver according to the present invention includes a first acquisition means for synchronizing the receiver at least with the signal of the first satellite, determining means for determining the phase of the information modulated in the signal of the first satellite on the basis of a moment of a bit change of an information data bit of the modulated signal, computing means for computing the difference in the time of propagation from the satellite to the receiver between the signal transmitted from the first satellite and the signal transmitted from the second satellite, second acquisition means for synchronizing the receiver with the signal of the second satellite on the basis of the difference in propagation time computed in the computing means, and integrating means for integrating epochs of a certain length from the received signal of the second satellite to improve demodulability of the signal.
The electronic device according to the present invention includes at least a first acquisition means for synchronizing the receiver at least with the signal of the first satellite, determining means for determining the phase of the information modulated in the signal of the first satellite on the basis of a moment of a bit change of an information data bit in the modulated signal, computing means for computing the difference in the time of propagation from the satellite to the receiver between the signal transmitted from the first satellite and the signal transmitted from the second satellite, second acquisition means for synchronizing the receiver with the signal of the second satellite on the basis of the difference in propagation time computed in the computing means, and integrating means for integrating epochs of a certain length from the received signal of the second satellite to improve demodulability of the signal.
Considerable advantages are achieved by the present invention when compared with methods and positioning receivers of prior art. When applying the method of the invention, positioning can also be performed when the signal strength of only one satellite is sufficiently strong. It is thus possible to determine the moment of bit change from the signals of the other satellites and to perform coherent integration of the signal for the time of several epochs without a bit change during the integration. The method of the invention can be used to reduce the time to first fix the receiver, e.g. when the receiver is turned on. The receiver can be synchronized very fast with the signal of one or more satellites that are sufficiently strong when received, and the synchronization with even weak signals of other satellites required can be significantly accelerated by the method of the invention. Moreover, in the method of the invention, the precise position of the receiver does not need to be computed, but a sufficient accuracy is a point in the vicinity of the receiver (in relation to the distance between the satellites and the receiver) whose position is known, e.g. the position of a base transceiver station. Thus, if navigation data, such as Ephemeris data and time data, has been received from at least one satellite in the receiver, no other information but this position data needs to be transmitted to the receiver. Furthermore, a sufficient accuracy for the reference clock is that the maximum error of the reference time is in the order of 2 min.